Memory cell having improved mechanical stability

ABSTRACT

Memory cells are described along with methods for manufacturing. A memory cell described herein includes a bottom electrode comprising a base portion and a pillar portion on the base portion, the pillar portion and the base portion having respective outer surfaces and the pillar portion having a width less than that of the base portion. A memory element is on a top surface of the pillar portion of the bottom electrode, and a top electrode is on the memory element. A dielectric spacer contacts the outer surface of the pillar portion, the outer surface of the base portion of the bottom electrode self-aligned with an outer surface of the dielectric spacer.

PARTIES TO A JOINT RESEARCH AGREEMENT

International Business Machines Corporation, a New York corporation;Macronix International Corporation, Ltd., a Taiwan corporation, andInfineon Technologies A.G., a German corporation, are parties to a JointResearch Agreement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high density memory devices based onprogrammable resistive materials, including phase change materials likechalcogenides, and to methods for manufacturing such devices.

2. Description of Related Art

Phase change based memory materials, like chalcogenide based materialsand similar materials, can be caused to change phase between anamorphous state and a crystalline state by application of electricalcurrent at levels suitable for implementation in integrated circuits.The generally amorphous state is characterized by higher electricalresistivity than the generally crystalline state, which can be readilysensed to indicate data. These properties have generated interest inusing programmable resistive material to form nonvolatile memorycircuits, which can be read and written with random access.

The change from the amorphous to the crystalline state is generally alower current operation. The change from crystalline to amorphous,referred to as reset herein, is generally a higher current operation,which includes a short high current density pulse to melt or breakdownthe crystalline structure, after which the phase change material coolsquickly, quenching the molten phase change material and allowing atleast a portion of the phase change material to stabilize in theamorphous state. It is desirable to minimize the magnitude of the resetcurrent used to cause transition of phase change material from thecrystalline state to the amorphous state. The memory cells using phasechange material include an “active region” in the bulk of the phasechange material of the cell in which the actual phase transition islocated. Techniques are applied to make the active region small, so thatthe amount of current needed to induce the phase change is reduced.Also, techniques are used to thermally isolate the active region in thephase change cell so that the resistive heating needed to induce thephase change is confined to the active region.

The magnitude of the current needed for reset can be reduced by reducingthe size of the phase change material element in the cell and/or thecontact area between electrodes and the phase change material, such thathigher current densities are achieved with small absolute current valuesthrough the phase change material element.

One direction of development has been toward forming small pores in anintegrated circuit structure, and using small quantities of programmableresistive material to fill the small pores. Patents illustratingdevelopment toward small pores include: Ovishinsky, “Multibit SingleCell Memory Element Tapered Contact”, U.S. Pat. No. 5,687,112, issuedNov. 11, 1997; Zahorik et al., “Method of Making Chalogenide [sic]Memory Device,” U.S. Pat. No. 5,789,277, issued Aug. 4, 1998; Doan etal., “Controllable Ovonic Phase-Change Semiconductor Memory Device andMethods of Fabricating the Same,” U.S. Pat. No. 6,150,253, issued Nov.21, 2000.

One approach to controlling the size of the active area in a phasechange cell is to devise very small electrodes for delivering current toa body of phase change material. This small electrode structure inducesphase change in the phase change material in a small area like the headof a mushroom, at the location of the contact. See, U.S. Pat. No.6,429,064 issued Aug. 6, 2002 to Wicker, “Reduced Contact Areas ofSidewall Conductor”; U.S. Pat. No. 6,462,353 issued Oct. 8, 2002 toGilgen, “Method for Fabricating a Small Area of Contact BetweenElectrodes”; U.S. Pat. No. 6,501,111 issued Dec. 31, 2002 to Lowrey,“Three-Dimensional (3D) Programmable Device”; U.S. Pat. No. 6,563,156issued Jul. 1, 2003 to Harshfield, “Memory Elements and Methods forMaking Same.”

One problem associated with manufacturing devices having very smallelectrodes arises because of poor adhesion of the very small electrodes,which can cause the bottom electrode to fall over during manufacturing.

A bottom electrode having an inverted T-shape has been proposed (U.S.patent application Ser. No. 12/016,840, filed 18 Jan. 2008 entitledMemory Cell with Memory Element Contacting an Inverted T-Shaped BottomElectrode) having a small contact area between the bottom electrode andmemory material, resulting in a small active region and reducing theamount of power needed for reset of the memory cell. The invertedT-shaped bottom electrode also improves the mechanical stability of thebottom electrode during manufacturing, thereby improving themanufacturing yield of such devices.

It is desirable therefore to provide a reliable method for manufacturinga memory cell structure with good control over the critical dimensionsof the bottom electrode while also addressing the mechanical stabilityissues of very small electrodes, which will work with high densityintegrated circuit memory devices.

SUMMARY

A memory cell described herein includes a bottom electrode comprising abase portion and a pillar portion on the base portion, the pillarportion and the base portion having respective outer surfaces and thepillar portion having a width less than that of the base portion. Amemory element is on a top surface of the pillar portion of the bottomelectrode, and a top electrode is on the memory element. A dielectricspacer contacts the outer surface of the pillar portion, the outersurface of the base portion of the bottom electrode self-aligned with anouter surface of the dielectric spacer.

A method for manufacturing a memory cell as described herein includesforming a memory core including a bottom electrode comprising a baseportion and a pillar portion on the base portion, the pillar portion andthe base portion having respective outer surfaces and the pillar portionhaving a width less than that of the base portion. The memory core alsoincludes a memory element on a top surface of the pillar portion of thebottom electrode, and a top electrode on the memory element. The methodalso includes forming a dielectric spacer contacting the outer surfaceof the pillar portion, the outer surface of the base portion of thebottom electrode self-aligned with an outer surface of the dielectricspacer.

A method for manufacturing a memory cell as described herein includesproviding a memory access layer having a top surface, the memory accesslayer including a conductive plug extending to the top surface of thememory access layer. A layer of bottom electrode material is formed onthe top surface of the memory access layer, a layer of memory materialis formed on the layer of bottom electrode material, a layer of topelectrode material is formed on the layer of memory material, and anetch mask is formed on the layer of top electrode material. Etching isperformed down through a portion of the layer of bottom electrodematerial using the etch mask. The etching forms a partially etched layerincluding a pillar of bottom electrode material and a multi-layer stackon the pillar of bottom electrode material. The multi-layer stackcomprises a memory element comprising memory material on the pillar ofbottom electrode material and a top electrode comprising top electrodematerial on the memory element. A layer of dielectric spacer material isformed on the partially etched layer and the multi-layer stack. Thedielectric spacer is anisotropically etched to form a dielectric spacercontacting an outer surface of the pillar of bottom electrode materialand an outer surface of the multi-layer stack. Etching is then performedon the partially etched layer using the dielectric spacer as an etchmask, thereby forming a bottom electrode comprising a base portion and apillar portion on the base portion.

The larger width of the base portion of the bottom electrodes describedherein provide better adhesion of the bottom electrode and reduce therisk of the bottom electrode falling over during manufacturing.Additionally, the design moves a locus of weakness (that is, the planewhere the narrower portion of the bottom electrode ends) away from theinterface between the bottom electrode and underlying structures towithin the bottom electrode material layer.

Other features, aspects and advantages of the present invention can beseen on review of the Figures, the detailed description, and the claimswhich follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a prior art “mushroom”memory cell.

FIG. 2 illustrates a cross-sectional view of a prior art “pillar-type”memory cell.

FIG. 3 illustrates a cross-sectional view of a memory cell havingimproved mechanical stability compared to the memory cell of FIG. 1.

FIG. 4 illustrates a cross-sectional view of a memory cell havingimproved mechanical stability compared to the memory cell of FIG. 2.

FIGS. 5-13 illustrate steps in a fabrication sequence for manufacturingthe memory cell illustrated in FIG. 3.

FIGS. 14-19 illustrate steps in a fabrication sequence for manufacturingthe memory cell illustrated in FIG. 4.

FIG. 20 is a simplified block diagram of an integrated circuit includinga memory array implemented using memory cells as described herein.

FIG. 21 is a portion of a memory array implemented using memory cellsdescribed herein.

DETAILED DESCRIPTION

The following description of the disclosure will typically be withreference to specific structural embodiments and methods. It is to beunderstood that there is no intention to limit the disclosure to thespecifically disclosed embodiments and methods but that the disclosuremay be practiced using other features, elements, methods andembodiments. Preferred embodiments are described to illustrate thepresent disclosure, not to limit its scope, which is defined by theclaims. Those of ordinary skill in the art will recognize a variety ofequivalent variations on the description that follows. Like elements invarious embodiments are commonly referred to with like referencenumerals.

A detailed description is provided with reference to FIGS. 1-21.

FIG. 1 illustrates a cross-sectional view of a prior art “mushroom”memory cell 100 having a layer of memory material 130 between a bottomelectrode 120 and a top electrode 140. A conductive plug 180 extendsthrough dielectric layer 170 to couple the memory cell 100 to underlyingaccess circuitry (not shown). A dielectric layer 190 surrounds thebottom electrode 120, and a dielectric layer 160 surrounds the topelectrode 140 and memory material 130. The bottom electrode 120 has awidth 125 less than the width 145 of the top electrode 140 and thememory material 130.

In operation, voltages on the plug 180 and the top electrode 140 caninduce current to flow from the plug 180 to the top electrode 140, orvice-versa, via the bottom electrode 120 and the memory material 130.

Due to the differences in the widths 125 and 145, in operation thecurrent density will be largest in the region of the memory material 130adjacent the bottom electrode 120, resulting in the active region 150 ofthe memory material 130 having a “mushroom” shape as shown in FIG. 1.

It is desirable to minimize the width 125 (which in some examples is adiameter) of the bottom electrode 120 so that higher current densitiesare achieved with small absolute current values through the memorymaterial 130.

However, attempts to reduce the width 125 can result in issues in theelectrical and mechanical reliability of the interface between thebottom electrode 120 and the plug 180 due to the small contact surfacetherebetween.

FIG. 2 illustrates a cross-sectional view of a prior art “pillar-type”memory cell 200. The memory cell 200 includes a multi-layer pillar 290comprising a bottom electrode 220, a pillar of memory material 230 onthe bottom electrode 220, and a top electrode 240 on the pillar ofmemory material 230. A dielectric layer 260 surrounds the pillar ofmemory material 230. A conductive plug 280 extends through dielectriclayer 270 to couple the memory cell 200 to underlying access circuitry(not shown).

As can be seen in the Figure the top and bottom electrodes 240, 220 havethe same width 245 as that of the pillar of memory material 230. Thus,the active region 250 can be spaced away from the top and bottomelectrodes 220, 240.

The multi-layer pillar 290 can be formed by sequentially forming a layerof bottom electrode material, a layer of memory material on the bottomelectrode material, a layer of top electrode material on the layer ofmemory material, and subsequently etching to form the pillar 290.However, problems have arisen in manufacturing such devices having smallwidths 245 and aggressive aspect ratios due to issues with undercutetching and/or overetching. Additionally, attempts to reduce the width245 can result in issues in the electrical and mechanical reliability ofthe interface between the bottom electrode 220 and the plug 280 due tothe small contact surface therebetween.

FIG. 3 illustrates a cross-sectional view of a memory cell 300addressing the issues describes above and resulting in improvedmechanical stability compared to the memory cell of FIG. 1. The memorycell 300 includes an inverted T-shaped bottom electrode 320 having abase portion 322 and a pillar portion 324 on the base portion 322. Thebase portion 322 has a first width 323 (which in some embodiments is adiameter) and the pillar portion 324 has a second width 325 (which insome embodiments is a diameter) less than the first width 323. Thelarger width 323 of the base portion 322 of the bottom electrode 320provides better mechanical stability for the bottom electrode 320.

The top surface of the pillar portion 324 contacts a memory element 330,the bottom electrode 320 coupling the memory element 330 to theconductive plug 380. The bottom electrode 320 may comprise, for example,TiN or TaN. TiN may be preferred in embodiments in which the memoryelement 330 comprises GST (discussed below) because is makes a goodcontact with GST, it is a common material used in semiconductormanufacturing, and it provides a good diffusion barrier at the highertemperatures at which GST transitions, typically in the 600-700° C.range. Alternatively, the bottom electrode may be TiAlN or TaAlN, orcomprises, for further examples, one or more elements selected from thegroup consisting of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, N, O, and Ruand combinations thereof.

The conductive plug 380 extends through dielectric layer 370 tounderlying access circuitry (not shown), the conductive plug 380comprising a refractory metal such as tungsten in the illustratedembodiment. Other metals that could be used include Ti, Mo, Al, Ta, Cu,Pt, Ir, La, Ni, and Ru. Other plug structures and materials can be usedas well.

A top electrode 340 contacts the memory element 330, the top electrode340 comprising a conductive material such as one or more of thematerials described above with reference to the bottom electrode 320.The top electrode 340 may comprise a portion of bit line. Alternatively,a conductive via (not shown) may couple the top electrode 340 to a bitline. The bottom electrode 320, memory element 330, and top electrode340 form a memory core of the memory cell 300.

A dielectric spacer 308 contacts the outer surface 326 of the pillarportion 324 and surrounds the pillar portion 324. During formation ofthe bottom electrode 320, the dielectric spacer 308 protects the baseportion 322 of the bottom electrode 320 from being etched. Thus, thebase portion 322 has an outside surface 321 self-aligned with theoutside surface 309 of the dielectric spacer 308.

Dielectric 310 surrounds the dielectric spacer 308 and the base portion322 of the bottom electrode 320. The dielectric spacer 308 anddielectric 310 each preferably comprise material resistant to diffusionof the phase change material of memory element 330, and in someembodiments the spacer 308 and dielectric 310 comprise the samematerial. Alternatively, the material of dielectric spacer 308 can bechosen, for example, for low thermal conductivity (discussed in moredetail below) and/or for use in selective processing (for exampleselective etching) during the formation of the memory cell 300(discussed in more detail with reference to FIGS. 5-13).

The dielectric spacer 308 may comprise an electrical insulator includingone or more elements selected from the group consisting of Si, Ti, Al,Ta, N, O, and C. In preferred devices, the dielectric materials have alow thermal conductivity, less than about 0.014 J/cm*K*sec. In otherpreferred embodiments, when memory element 330 is made from a phasechange material, the dielectric spacer 308 comprises material having athermal conductivity less than that of the amorphous state of the phasechange material, or less than about 0.003 J/cm*K*sec for a phase changematerial comprising GST. Representative thermally insulating materialsinclude materials that are a combination of the elements silicon (Si),carbon (C) oxygen (O), fluorine (F), and hydrogen (H). Examples ofthermally insulating materials which are candidates for use for thethermally insulating dielectric 308 include SiO₂, SiCOH, polyimide,polyamide, and fluorocarbon polymers. Other examples of materials whichare candidates for use for the thermally insulating dielectric materialsinclude fluorinated SiO₂, silsesquioxane, polyarylene ethers, parylene,fluoro-polymers, fluorinated amorphous carbon, diamond like carbon,porous silica, mesoporous silica, porous silsesquioxane, porouspolyimide, and porous polyarylene ethers. In other embodiments, thethermally insulating structure comprises a gas-filled void for thermalinsulation. A single layer or combination of layers within thedielectric materials can provide thermal and electrical insulation. Inthe manufacturing process described in FIGS. 5-12 below, the dielectricspacer 308 may also act as an etch mask, and thus may be chosen for itsselective etching characteristics.

A dielectric 360 surrounds the memory element 330, and in someembodiments the dielectric 360 comprises the same material as that ofthe dielectric 310.

In operation, voltages on the plug 380 and the top electrode 340 caninduce a current to flow from the plug 380 to the top electrode 340, orvice versa, via the bottom electrode 320 and the memory element 340.

The active region 350 is the region of the memory element 330 in whichthe memory material is induced to change between at least two solidphases. As can be appreciated, the active region 350 can be madeextremely small in the illustrated structure, thus reducing themagnitude of the current needed to induce a phase change. The thickness332 of the memory material of the memory element 330 can be establishedusing a thin film deposition technique of memory material on the bottomelectrode 320. In some embodiments the thickness 332 is less than orequal to about 100 nm, for example being between 10 and 100 nm.Furthermore, the width or diameter 325 of the pillar portion 324 of thebottom electrode 320 is less than the width or diameter 334 of thememory element 330 and is preferably less than a minimum feature sizefor a process, typically a lithographic process, used to form the memorycell 300. The small pillar portion 324 of the bottom electrode 320concentrates current density in the portion of the memory element 330adjacent the bottom electrode 320, thereby reducing the magnitude of thecurrent needed to induce a phase change in the active region 350.Additionally, the dielectric spacer 308 preferably provides some thermalisolation to the active region 350, which also helps to reduce theamount of current necessary to induce a phase change.

The bottom electrode 320 having an inverse T-shape adds mechanicalstability in two ways. First, the increased area between the bottomelectrode 320 and the plug 380 increases the strength of the unit as awhole. Second, the design moves a locus of weakness (that is, the planewhere the narrower portion of the bottom electrode 320 ends) away fromthe interface between the bottom electrode 320 and the plug 380 towithin a monolithic material layer (the bottom electrode 320). Inaddition, the electrical contact resistance between the bottom electrode320 and the underlying conductive plug 380 is reduced due to theincreased contact area.

FIG. 4 illustrates a cross-sectional view of a second memory cell 400having improved mechanical stability compared to the memory cell of FIG.2. The memory cell includes an inverted T-shaped bottom electrode 420having a base portion 422 and a pillar portion 424 on the base portion422. The base portion 422 has a first width 423 (which in someembodiments is a diameter) and the pillar portion 424 has a second width425 (which in some embodiments is a diameter) less than the first width423. The larger width 423 of the base portion 422 of the bottomelectrode 420 provides better mechanical stability to the bottomelectrode 420.

The top surface of the pillar portion 424 contacts a pillar memoryelement 430 comprising memory material, the bottom electrode 420coupling the memory element 430 to the conductive plug 380. The bottomelectrode 420, for example, may comprise any of the materials of thebottom electrode 320 discussed above with reference to FIG. 3.

As can be seen in FIG. 4, the width of the pillar memory element 430 andthe width of a top electrode 440 are substantially equal to the width425 of the pillar portion 424 of the bottom electrode 420. As usedherein, the term “substantially” is intended to accommodatemanufacturing tolerances. Thus, the pillar memory element 430 has anactive region 450 that can be spaced away from both the top and bottomelectrodes 440, 430. The top electrode 440 may comprise, for example,any of the materials of the top electrode 340 described above withreference to FIG. 3. The bottom electrode 420, memory element 430, andtop electrode 440 form a memory core of the memory cell 400.

A dielectric spacer 408 contacts the outer surface 426 of the pillarportion 424 and surrounds the pillar portion 424. The dielectric spacer408 can be used as an etch mask during the formation of the base portion422 of the bottom electrode 420. Accordingly, the base portion 422 ofthe bottom electrode 420 has an outer surface 421 self-aligned with theouter surface 409 of the dielectric spacer 408.

The dielectric spacer 408 may comprise, for example, any of thematerials discussed above with reference to the dielectric spacer 308 ofFIG. 3. The dielectric spacer 408 preferably comprises materialresistant to diffusion of the phase change material of memory element430. In the manufacturing process described in FIGS. 14-19 below thedielectric spacer 408 acts as an etch mask used in the formation of thebottom electrode 420, and thus preferably comprises material that can beselectively etched.

A dielectric 410 surrounds the dielectric spacer 408 and the baseportion 422 of the bottom electrode 420. In some embodiments thedielectric spacer 408 and dielectric 410 each comprise the samematerial. Alternatively, the material of dielectric spacer 408 can bechosen, for example, for low thermal conductivity (discussed in moredetail below) and/or for use in selective processing (for exampleselective etching) during the formation of the memory cell 400(discussed in more detail below with reference to FIGS. 14-19).

In operation, voltages on the plug 380 and the top electrode 440 caninduce a current to flow from the plug 380 to the top electrode 440, orvice versa, via the bottom electrode 420 and the memory element 440.

The active region 450 is the region of the memory element 430 in whichthe memory material is induced to change between at least two solidphases. As can be appreciated the active region can be made extremelysmall in the illustrated structure, thus reducing the magnitude of thecurrent needed to induce a phase change. The thickness 432 of the memorymaterial of the memory element 430 can be established using a thin filmdeposition technique of memory material on the bottom electrode 420. Insome embodiments the thickness 432 is less than or equal to about 100nm, for example being between 10 and 100 nm. Furthermore, the width ordiameter 425 of the pillar portion 424 of the bottom electrode 420 isequal to that of the memory element 430 and the top electrode 440. Thus,the active region 450 can be spaced away from the top and bottomelectrodes 440, 420 and the remaining portions of the memory element 430can provide some thermal isolation to the active region 450.Additionally, the width 424 is preferably less than a minimum featuresize for a process, typically a lithographic process, used to form thememory cell 400. Also, the dielectric spacer 408 preferably comprises amaterial which provides some thermal isolation to the active region 450,which also helps to reduce the amount of current necessary to induce aphase change.

The bottom electrode 420 provides additional mechanical stability andimproved performance of the interface between the bottom electrode 420and the plug 380 for the same reasons as described above with referenceto FIG. 3.

Embodiments of the memory cells 300, 400 include phase change basedmemory materials, including chalcogenide based materials and othermaterials, for the memory elements 330, 430 respectively. Chalcogensinclude any of the four elements oxygen (0), sulfur (S), selenium (Se),and tellurium (Te), forming part of group VIA of the periodic table.Chalcogenides comprise compounds of a chalcogen with a moreelectropositive element or radical. Chalcogenide alloys comprisecombinations of chalcogenides with other materials such as transitionmetals. A chalcogenide alloy usually contains one or more elements fromgroup IVA of the periodic table of elements, such as germanium (Ge) andtin (Sn). Often, chalcogenide alloys include combinations including oneor more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag).Many phase change based memory materials have been described intechnical literature, including alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te,Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te,Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Tealloys, a wide range of alloy compositions may be workable. Thecompositions can be characterized as Te_(a)Ge_(b)Sb_(100−(a+b)). Oneresearcher has described the most useful alloys as having an averageconcentration of Te in the deposited materials well below 70%, typicallybelow about 60% and ranged in general from as low as about 23% up toabout 58% Te and most preferably about 48% to 58% Te. Concentrations ofGe were above about 5% and ranged from a low of about 8% to about 30%average in the material, remaining generally below 50%. Most preferably,concentrations of Ge ranged from about 8% to about 40%. The remainder ofthe principal constituent elements in this composition was Sb. Thesepercentages are atomic percentages that total 100% of the atoms of theconstituent elements. (Ovshinsky U.S. Pat. No. 5,687,112 patent, cols.10-11.) Particular alloys evaluated by another researcher includeGe₂Sb₂Te₅, GeSb₂Te₄ and GeSb₄Te₇ (Noboru Yamada, “Potential of Ge—Sb—TePhase-Change Optical Disks for High-Data-Rate Recording”, SPIE v.3109,pp. 28-37 (1997).) More generally, a transition metal such as chromium(Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum(Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te toform a phase change alloy that has programmable resistive properties.Specific examples of memory materials that may be useful are given inOvshinsky '112 at columns 11-13, which examples are hereby incorporatedby reference.

Chalcogenides and other phase change materials are doped with impuritiesin some embodiments to modify conductivity, transition temperature,melting temperature, and other properties of memory elements using thedoped chalcogenides. Representative impurities used for dopingchalcogenides include nitrogen, silicon, oxygen, silicon dioxide,silicon nitride, copper, silver, gold, aluminum, aluminum oxide,tantalum, tantalum oxide, tantalum nitride, titanium and titanium oxide.See, e.g., U.S. Pat. No. 6,800,504, and U.S. Patent ApplicationPublication No. U.S. 2005/0029502.

Phase change alloys are capable of being switched between a firststructural state in which the material is in a generally amorphous solidphase, and a second structural state in which the material is in agenerally crystalline solid phase in its local order in the activechannel region of the cell. These alloys are at least bistable. The termamorphous is used to refer to a relatively less ordered structure, moredisordered than a single crystal, which has the detectablecharacteristics such as higher electrical resistivity than thecrystalline phase. The term crystalline is used to refer to a relativelymore ordered structure, more ordered than in an amorphous structure,which has detectable characteristics such as lower electricalresistivity than the amorphous phase. Typically, phase change materialsmay be electrically switched between different detectable states oflocal order across the spectrum between completely amorphous andcompletely crystalline states. Other material characteristics affectedby the change between amorphous and crystalline phases include atomicorder, free electron density and activation energy. The material may beswitched either into different solid phases or into mixtures of two ormore solid phases, providing a gray scale between completely amorphousand completely crystalline states. The electrical properties in thematerial may vary accordingly.

Phase change alloys can be changed from one phase state to another byapplication of electrical pulses. It has been observed that a shorter,higher amplitude pulse tends to change the phase change material to agenerally amorphous state. A longer, lower amplitude pulse tends tochange the phase change material to a generally crystalline state. Theenergy in a shorter, higher amplitude pulse is high enough to allow forbonds of the crystalline structure to be broken and short enough toprevent the atoms from realigning into a crystalline state. Appropriateprofiles for pulses can be determined, without undue experimentation,specifically adapted to a particular phase change alloy. In followingsections of the disclosure, the phase change material is referred to asGST, and it will be understood that other types of phase changematerials can be used. A material useful for implementation of a PCRAMdescribed herein is Ge₂Sb₂Te₅.

Other programmable resistive memory materials may be used in otherembodiments of the invention, including N2 doped GST, Ge_(x)Sb_(y), orother material that uses different crystal phase changes to determineresistance; Pr_(x)Ca_(y)MnO₃, Pr_(x)Sr_(y)MnO₃, ZrO_(x), or othermaterial that uses an electrical pulse to change the resistance state;7,7,8,8-tetracyanoquinodimethane (TCNQ), methanofullerene 6,6-phenylC61-butyric acid methyl ester (PCBM), TCNQ-PCBM, Cu-TCNQ, Ag-TCNQ,C₆₀-TCNQ, TCNQ doped with other metal, or any other polymer materialthat has a bistable or multi-stable resistance state controlled by anelectrical pulse.

An exemplary method for forming chalcogenide material usesPVD-sputtering or magnetron-sputtering method with source gas(es) of Ar,N₂, and/or He, etc. at the pressure of 1 mTorr˜100 mTorr. The depositionis usually done at room temperature. A collimator with an aspect ratioof 1˜5 can be used to improve the fill-in performance. To improve thefill-in performance, a DC bias of several tens of volts to severalhundreds of volts is also used. On the other hand, the combination of DCbias and the collimater can be used simultaneously.

A post-deposition annealing treatment in a vacuum or in an N₂ ambient isoptionally performed to improve the crystallize state of chalcogenidematerial. The annealing temperature typically ranges from 100° C. to400° C. with an anneal time of less than 30 minutes.

The thickness of chalcogenide material depends on the design of cellstructure. In general, a chalcogenide material with thickness of higherthan 1.5 nm can have a phase change transition so that the materialexhibits at least two stable resistance states.

FIGS. 5-13 illustrate steps in a fabrication sequence for manufacturingthe memory cell illustrated in FIG. 3.

FIG. 5 illustrates a cross-sectional view of a first step of providing amemory access layer 500 having a top surface 502 and includingconductive plug 380 extending through dielectric 370 to the top surface502. The memory access layer 500 can be formed by standard processes asknown in the art, and the configuration of elements of the memory accesslayer 500 depends upon the array configuration in which the memory cellsdescribed herein are implemented. Generally, the memory access layer 500may include access devices such as transistors, word lines and sourcelines, conductive plugs, and doped regions within a semiconductorsubstrate.

Next, a bottom electrode material layer 620 is formed on the top surface502 of the memory access layer 500 and an etch mask comprising a maskelement 630 is formed on the bottom electrode material layer 620,resulting in the structure illustrated in FIG. 6. The bottom electrodematerial layer 620 may comprise one or more layers of the materialsdescribed above with reference to the bottom electrode 320 of FIG. 3.

The mask element 630 can be formed by patterning a layer of photoresiston the layer 620 using a lithographic process, and then trimming thepatterned photoresist to form the mask element 630 having asub-lithographic width 632, for example being less than 50 nm in someembodiments. Photoresist trimming is applied, for example, using anoxygen plasma to isotropically etch the photoresist and reduces thedimension of the photoresist in both the vertical and horizontaldimensions. In an alternative embodiment, a hard mask layer such as alow temperature deposited layer of SiN or SiO₂ can be patterned usingphotolithography, followed by trimming using an isotropic wet etch, suchas dilute HF for silicon dioxide or hot phosphoric acid for siliconnitride, or isotropic fluorine or HBr based reactive ion etching.

Next, anisotropic timing mode etching is performed on the bottomelectrode material layer 620 using the mask element 630 as an etch mask,thereby forming partially etched layer 700 comprising the remainingmaterial of the bottom electrode material layer 620. The mask element630 is then removed, resulting in the structure illustrated in FIG. 7.The layer 700 includes a pillar 710 having a sidewall 711 and underlyingthe location of mask element 630, the pillar 710 not extending all theway through the layer 700. In one example the pillar 710 has a height712 of about 40 to 120 nm, for example being about 60 nm high. Theremaining portion of the layer 700 has a thickness 720 sufficient toprovide the mechanical integrity discussed above.

The timing mode etching may be done, for example, using a chlorine orfluorine based Reactive Ion Etching RIE process. In one embodiment, TiNis anisotropically etched using a chlorine based RIE, and in anotherembodiment a similar chlorine process is used to anisotropically etchTaN.

Next, a conformal layer 800 of dielectric spacer material is formed onthe structure illustrated in FIG. 7 having a thickness 810, resulting inthe structure illustrated in FIG. 8. In the illustrated embodiment layer800 comprises silicon dioxide and is formed using chemical vapordeposition CVD. Other materials chosen for their etch chemistry and theability to grow conformal layers on high aspect ratio structures couldalso be used for the layer 800. Also, other procedures, such as atomiclayer deposition, physical layer deposition, low-pressure chemical vapordeposition (LPCVD) of high density plasma chemical vapor deposition(HDPCVD) could be used to deposit the layer 800, depending on thematerials and geometries involved.

Next, anisotropic etching is performed on the layers 700 and 800 of thestructure illustrated in FIG. 8, resulting in the structure illustratedin FIG. 9 having an inverse T-shaped bottom electrode 320 and adielectric spacer 308.

In the illustration of FIG. 9 the dielectric spacer 308 and the bottomelectrode 320 have substantially co-planar top surfaces, although itwill be understood that alternatively the top surfaces of the dielectricspacer 308 and the bottom electrode 320 may be other than co-planarafter the anisotropic etching process. The relative location of the topsurface of the dielectric spacer 308 to that of the bottom electrode 320depends upon many factors including the thicknesses 720 and 810, thematerials of layers 800 and 700, and the etch chemistry used.

During the anisotropic etching the dielectric spacer 308 protects thebase portion 322 of the bottom electrode, and thus the base portion 322of the bottom electrode 320 has an outer surface 321 self-aligned withthe outer surface 309 of the dielectric spacer 308.

The anisotropic etching may be performed using a single etchingchemistry to etch both layers 700 and 800. Alternatively, theanisotropic etching may comprise a first etch chemistry toanisotropically etch layer 800 to form dielectric spacer 308, and asecond etch chemistry to etch layer 700 to form the bottom electrode 320using the spacer 308 as an etch mask.

Next, a layer 310 of dielectric material is formed on the structureillustrated in FIG. 9 and planarized, resulting in the structureillustrated in FIG. 10 having a top surface 1000. Layer 310 is formed inone embodiment by high-density plasma chemical vapor deposition (HDPCVD), followed by chemical-mechanical polishing (CMP) to expose thebottom electrode 320. In one embodiment the dielectric 310 comprisessilicon dioxide formed by chemical vapor deposition using a silane andO₂ chemistry at 400 to 450 C. For embodiments in which the dielectric310 is silicon nitride, a similar process is used with ammonia added tothe silane. For oxynitride, one should use ammonia, silane and oxygen.The dielectric 310 may comprise silicon oxides, silicon nitrides andother insulating materials, preferably having good thermal as well aselectrical insulating properties.

A memory material layer 1100 is then formed on the top surface 1000 anda top electrode material layer 1110 is formed on the memory layer 1100,resulting in the structure illustrated in FIG. 11. The memory layer 1100and the top electrode layer 1110 can each be less than 100 nm thick, forexample being between about 10 and 100 nm thick.

Next, the memory layer 1100 and the top electrode layer 1110 arepatterned to form a multi-layer stack comprising a memory element 330and a top electrode 340, resulting in the structure illustrated in FIG.12. Alternatively, the memory layer 1100 and the top electrode layer1110 may be patterned to form bit lines from the top electrode layer1110. The bottom electrode 320, memory element 330, and top electrode340 form a memory core.

Next, another dielectric layer 360 is formed on the structureillustrated in FIG. 12 and planarized, for example using CMP, resultingin the structure illustrated in FIG. 13.

FIGS. 14-19 illustrate steps in a fabrication sequence for manufacturingthe memory cell illustrated in FIG. 4.

FIG. 14 illustrates a cross-sectional view of forming a bottom electrodematerial layer 1400 on the top surface 502 of the access layer 500 ofFIG. 5, forming a memory layer 1410 on the bottom electrode materiallayer 1400, forming a top electrode material layer 1420 on the memorylayer 1410, and forming an etch mask comprising a mask element 1430 onthe top electrode material layer 1420. The bottom electrode materiallayer 1400 may comprise one or more layers of the materials describedabove with reference to the bottom electrode 320 of FIG. 3.

The mask element 1430 can be formed by patterning a layer of photoresiston the layer 1420 using a lithographic process, and then trimming thepatterned photoresist to form the mask element 1430 having asub-lithographic width 1432, for example being less than 50 nm in someembodiments. Photoresist trimming is applied, for example, using anoxygen plasma to isotropically etch the photoresist and reduces thedimension of the photoresist in both the vertical and horizontaldimensions. In an alternative embodiment, a hard mask layer such as alow temperature deposited layer of SiN or SiO₂ can be patterned usingphotolithography, followed by trimming using an isotropic wet etch, suchas dilute HF for silicon dioxide or hot phosphoric acid for siliconnitride, or isotropic fluorine or HBr based reactive ion etching.

Next, anisotropic timing mode etching is performed using the maskelement 1430 as an etch mask, thereby forming a partially etched layer1500 comprising the remaining portion of layer 1400 and including pillar1510 of the bottom electrode material of layer 1400, and forming amulti-layer stack 1530 on pillar 1510. The multi-layer stack 1530comprises memory element 430 on pillar 1510, and top electrode 440 onthe memory element 430. The mask element 1430 is then removed, resultingin the structure illustrated in FIG. 15. The layer 1500 includes apillar 1510 that does not extend all the way through the layer 1500. Themulti-layer stack 1530 has a width substantially equal to that of pillar1510. The remaining portion of the layer 1500 has a thickness 1520sufficient to provide the mechanical integrity discussed above.

The timing mode etching may be done using a chlorine or fluorine basedReactive Ion Etching RIE process. In one embodiment, TiN isanisotropically etched using a chlorine based RIE, and in anotherembodiment a similar chlorine process is used to anisotropically etchTaN.

Next, a conformal layer 1600 of dielectric spacer material is formed onthe structure illustrated in FIG. 15 having a thickness 1610, resultingin the structure illustrated in FIG. 16. In the illustrated embodimentlayer 1600 comprises silicon dioxide and is formed using chemical vapordeposition CVD. Other materials chosen for their etch chemistry and theability to grow conformal layers on high aspect ratio structures couldalso be used for the layer 1600. Also, other procedures, such as atomiclayer deposition, physical layer deposition, low-pressure chemical vapordeposition (LCPVD) of high density plasma chemical vapor deposition(HDPCVD) could be used to deposit the layer 1600, depending on thematerials and geometries involved.

Next, anisotropic etching is performed on the layer 1600 to formdielectric spacer 408, resulting in the structure illustrated in FIG.17.

Next, the layer 1400 is etched using the dielectric spacer 408 as anetch mask, resulting in the structure illustrated in FIG. 18 having aninverted T-shaped bottom electrode 420 including a base portion 422 anda pillar portion 424 on the base portion 422. The bottom electrode 420,memory element 430, and top electrode 440 form a memory core.

Since the layer 1400 is etched using the dielectric spacer 408 as anetch mask, the base portion 422 of the bottom electrode 420 has an outersurface 421 self-aligned with the outer surface 409 of the dielectricspacer 408.

In FIG. 18 the bottom electrode 420 comprises material that can beselectively etched from that of material of the top electrode 440, andthus the top electrode 440 is also used as an etch mask during theetching of layer 1400. Alternatively, the top electrode 440 may also beetched.

In an alternative embodiment a suitable hard mask layer is deposited onthe top electrode material layer 1420 of FIG. 14 and the mask element1430 is formed on the hard mask layer, and the subsequent etching stepof FIG. 15 results in a portion of the hard mask layer remaining on themulti-layer stack 1530. This remaining portion of the hard mask layerprotects the top electrode 440 during the etching of layer 1400.

Next, a layer 410 of dielectric material is formed on the structureillustrated in FIG. 18 and planarized, resulting in the structureillustrated in FIG. 19 having a top surface 1900. Layer 410 is formed inone embodiment by high-density plasma chemical vapor deposition (HDPCVD), followed by chemical-mechanical polishing (CMP) to expose the topelectrode 440. In one embodiment the dielectric 410 comprises silicondioxide formed by chemical vapor deposition using a silane and O₂chemistry at 400 to 450 C. For embodiments in which the dielectric 410is silicon nitride, a similar process is used with ammonia added insteadof silane. For oxynitride, one should use ammonia, silane and oxygen.The dielectric 410 may comprise silicon oxides, silicon nitrides andother insulating materials, preferably having good thermal as well aselectrical insulating properties.

Next, subsequent further processing such as forming a bit line coupledto the memory cell can then be done.

FIG. 20 is a simplified block diagram of an integrated circuit 10including a memory array 12 implemented using memory cells as describedherein with reference to FIGS. 3 or 4. A word line decoder 14 is coupledto and in electrical communication with a plurality of word lines 16. Abit line (column) decoder 18 is in electrical communication with aplurality of bit lines 20 to read data from, and write data to, thephase change memory cells (not shown) in array 12. Addresses aresupplied on bus 22 to word line decoder and drivers 14 and bit linedecoder 18. Sense amplifiers and data-in structures in block 24 arecoupled to bit line decoder 18 via data bus 26. Data is supplied via adata-in line 28 from input/output ports on integrated circuit 10, orfrom other data sources internal or external to integrated circuit 10,to data-in structures in block 24. Other circuitry 30 may be included onintegrated circuit 10, such as a general purpose processor or specialpurpose application circuitry, or a combination of modules providingsystem-on-a-chip functionality supported by array 12. Data is suppliedvia a data-out line 32 from the sense amplifiers in block 24 toinput/output ports on integrated circuit 10, or to other datadestinations internal or external to integrated circuit 10.

A controller 34 implemented in this example, using a bias arrangementstate machine, controls the application of bias arrangement supplyvoltages 36, such as read, program, erase, erase verify and programverify voltages. Controller 34 may be implemented using special-purposelogic circuitry as known in the art. In alternative embodiments,controller 34 comprises a general-purpose processor, which may beimplemented on the same integrated circuit to execute a computer programto control the operations of the device. In yet other embodiments, acombination of special-purpose logic circuitry and a general-purposeprocessor may be utilized for implementation of controller 34.

As shown in FIG. 21 each of the memory cells of array 12 includes anaccess transistor (or other access device such as a diode), four ofwhich are shown as 38, 40, 42 and 44, a phase change element shown as46, 48, 50 and 52, and inverted T-shaped bottom electrode shown as 47,49, 51, and 53. Sources of each of the access transistors 38, 40, 42 and44 are connected in common to a source line 54 that terminates in asource line termination 55. In another embodiment the source lines ofthe select devices are not electrically connected, but independentlycontrollable. A plurality of word lines including word lines 56 and 58extend parallel along a first direction. Word lines 56 and 58 are inelectrical communication with word line decoder 14. The gates of accesstransistors 38 and 42 are connected to a common word line, such as wordline 56, and the gates of access transistors 40 and 44 are connected incommon to word line 58. A plurality 20 of bit lines including bit lines60 and 62 have one end of phase change elements 46 and 48 connected tobit line 60. Specifically, phase change element 46 is connected betweenthe drain of access transistor 38 and bit line 60, and phase changeelement 48 is connected between the drain of access transistor 48 andbit line 60. Similarly, phase change element 50 is connected between thedrain of access transistor 42 and bit line 62, and phase change element52 is connected between the drain of access transistor 44 and bit line62. It should be noted that four memory cells are shown for convenienceof discussion and in practice array 12 may comprise thousands tomillions of such memory cells. Also, other array structures may be used,e.g. the phase change memory element is connected to source.Additionally, instead of MOS transistors, bipolar transistors or diodesmay be used as an access device.

The invention has been described with reference to phase changematerials. However, other memory materials, also sometimes referred toas programmable materials, can also be used. As used in thisapplication, memory materials are those materials having electricalproperties, such as resistance, that can be changed by the applicationof energy; the change can be a stepwise change or a continuous change ora combination thereof.

While the present disclosure is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations will occurto those skilled in the art, which modifications and combinations willbe within the spirit of the disclosure and the scope of the followingclaims. What is claimed is:

1. A memory cell comprising: a bottom electrode comprising a baseportion and a pillar portion on the base portion, the pillar portion andthe base portion having respective outer surfaces and the pillar portionhaving a width less than that of the base portion; a memory element on atop surface of the pillar portion of the bottom electrode; a topelectrode on the memory element; and a dielectric spacer contacting theouter surface of the pillar portion, the outer surface of the baseportion of the bottom electrode self-aligned with an outer surface ofthe dielectric spacer.
 2. The memory cell of claim 1, wherein the widthof the pillar portion is less than a minimum feature size for alithographic process used to form the memory cell.
 3. The memory cell ofclaim 1, wherein the top and bottom electrode each comprise an elementchosen from a group consisting of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni,N, O, and Ru and combinations thereof.
 4. The memory cell of claim 1,wherein the memory material comprises a combination of two or morematerials from a group consisting of Ge, Sb, Te, Se, In, Ti, Ga, Bi, Sn,Cu, Pd, Pb, Ag, S, Si, O, P, As, N and Au.
 5. The memory cell of claim1, wherein the memory element and the top electrode form a multi-layerstack having a width greater than that of the pillar portion of thebottom electrode.
 6. The memory cell of claim 1, wherein the memoryelement and the top electrode have respective widths substantially equalto that of the pillar portion of the bottom electrode.
 7. The memorycell of claim 6, wherein the dielectric spacer contacts an outer surfaceof the memory element and an outer surface of the top electrode.
 8. Thememory cell of claim 7, wherein the dielectric spacer surrounds thepillar portion of the bottom electrode and the memory element.
 9. Thememory cell of claim 1, wherein the dielectric spacer has a thermalconductivity less than that of the memory material.
 10. A method formanufacturing a memory cell, the method comprising: forming a memorycore including: a bottom electrode comprising a base portion and apillar portion on the base portion, the pillar portion and the baseportion having respective outer surfaces and the pillar portion having awidth less than that of the base portion; a memory element on a topsurface of the pillar portion of the bottom electrode; and a topelectrode on the memory element; and forming a dielectric spacercontacting the outer surface of the pillar portion, the outer surface ofthe base portion of the bottom electrode self-aligned with an outersurface of the dielectric spacer.
 11. The method of claim 10, whereinthe width of the pillar portion is less than a minimum feature size fora lithographic process used to form the memory cell.
 12. The method ofclaim 10, wherein the step of forming a memory core comprises: forming alayer of memory material on the bottom electrode; forming a layer of topelectrode material on the layer of memory material; and patterning thelayer of memory material and the layer of top electrode material to forma multi-layer stack comprising the memory element and the top electrode.13. The method of claim 12, wherein the multi-layer stack has a widthgreater than that of the pillar portion of the bottom electrode.
 14. Themethod of claim 10, wherein the step of forming a memory core and thestep of forming a dielectric spacer comprise: providing a memory accesslayer having a top surface, the memory access layer including aconductive plug extending to the top surface of the memory access layer;forming a layer of bottom electrode material on the top surface of thememory access layer; forming an etch mask overlying the layer of bottomelectrode material; etching through a portion of the layer of bottomelectrode material using the etch mask, thereby forming a partiallyetched layer including a pillar of bottom electrode material; forming alayer of dielectric spacer material overlying the partially etchedlayer; and etching the layer of dielectric spacer material and thepartially etched layer to form the dielectric spacer and the bottomelectrode.
 15. The method of claim 14, wherein the etching the layer ofdielectric spacer material and the partially etched layer comprise:anisotropically etching the layer of dielectric spacer material to formthe dielectric spacer; and etching the partially etched layer using thedielectric spacer as an etch mask, thereby forming the bottom electrode.16. The method of claim 10, wherein the step of forming a memory corecomprises: providing a memory access layer having a top surface, thememory access layer including a conductive plug extending to the topsurface of the memory access layer; forming a layer of bottom electrodematerial on the top surface of the memory access layer; forming a layerof memory material on the layer of bottom electrode material; forming alayer of top electrode material on the layer of memory material; formingan etch mask on the layer of top electrode material; etching downthrough a portion of the layer of bottom electrode material using theetch mask, thereby forming a partially etched layer including a pillarof bottom electrode material and a multi-layer stack on the pillar ofbottom electrode material, the multi-layer stack comprising a memoryelement comprising memory material on the pillar of bottom electrodematerial and a top electrode comprising top electrode material on thememory element.
 17. The method of claim 16, wherein the multi-layerstack has a width substantially equal to that of the pillar portion ofthe bottom electrode.
 18. A method for manufacturing a memory cell, themethod comprising: providing a memory access layer having a top surface,the memory access layer including a conductive plug extending to the topsurface of the memory access layer; forming a layer of bottom electrodematerial on the top surface of the memory access layer; forming a layerof memory material on the layer of bottom electrode material; forming alayer of top electrode material on the layer of memory material; formingan etch mask on the layer of top electrode material; etching downthrough a portion of the layer of bottom electrode material using theetch mask, thereby forming a partially etched layer including a pillarof bottom electrode material and a multi-layer stack on the pillar ofbottom electrode material, the multi-layer stack comprising a memoryelement comprising memory material on the pillar of bottom electrodematerial and a top electrode comprising top electrode material on thememory element; forming a layer of dielectric spacer material on thepartially etched layer and the multi-layer stack; anisotropicallyetching the layer of dielectric spacer material to form a dielectricspacer contacting an outer surface of the pillar of bottom electrodematerial and an outer surface of the multi-layer stack; and etching thepartially etched layer using the dielectric spacer as an etch mask,thereby forming a bottom electrode comprising a base portion and apillar portion on the base portion.